Tag Archives: COVID-19

An Overlooked Variable May Be the Key to the Pandemic

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A writer for The Atlantic argues that there’s “a potential, overlooked way of understanding this pandemic that would help answer [questions about it], reshuffle many of the current heated arguments, and, crucially, help us get the spread of COVID-19 under control”:

By now many people have heard about R0—the basic reproductive number of a pathogen, a measure of its contagiousness on average. But unless you’ve been reading scientific journals, you’re less likely to have encountered k, the measure of its dispersion. The definition of k is a mouthful, but it’s simply a way of asking whether a virus spreads in a steady manner or in big bursts, whereby one person infects many, all at once. After nine months of collecting epidemiological data, we know that this is an overdispersed pathogen, meaning that it tends to spread in clusters, but this knowledge has not yet fully entered our way of thinking about the pandemic—or our preventive practices.

The now-famed R0 (pronounced as “r-naught”) is an average measure of a pathogen’s contagiousness, or the mean number of susceptible people expected to become infected after being exposed to a person with the disease. If one ill person infects three others on average, the R0 is three. This parameter has been widely touted as a key factor in understanding how the pandemic operates. News media have produced multiple explainers and visualizations for it. . . . . Dashboards track its real-time evolution, often referred to as R or Rt, in response to our interventions. . .

Unfortunately, averages aren’t always useful for understanding the distribution of a phenomenon, especially if it has widely varying behavior. If Amazon’s CEO, Jeff Bezos, walks into a bar with 100 regular people in it, the average wealth in that bar suddenly exceeds $1 billion. . . .Clearly, the average is not that useful a number to understand the distribution of wealth in that bar, or how to change it. . . . Meanwhile, if the bar has a person infected with COVID-19, and if it is also poorly ventilated and loud, causing people to speak loudly at close range, almost everyone in the room could potentially be infected—a pattern that’s been observed many times since the pandemic begin, and that is similarly not captured by R. That’s where the dispersion comes in.

There are COVID-19 incidents in which a single person likely infected 80 percent or more of the people in the room in just a few hours. But, at other times, COVID-19 can be surprisingly much less contagious. Overdispersion and super-spreading of this virus are found in research across the globe. A growing number of studies estimate that a majority of infected people may not infect a single other person. A recent paper found that in Hong Kong, which had extensive testing and contact tracing, about 19 percent of cases were responsible for 80 percent of transmission, while 69 percent of cases did not infect another person.

This finding is not rare: Multiple studies from the beginning have suggested that as few as 10 to 20 percent of infected people may be responsible for as much as 80 to 90 percent of transmission, and that many people barely transmit it.

This highly skewed, imbalanced distribution means that an early run of bad luck with a few super-spreading events, or clusters, can produce dramatically different outcomes even for otherwise similar countries. Scientists looked globally at known early-introduction events, in which an infected person comes into a country, and found that in some places, such imported cases led to no deaths or known infections, while in others, they sparked sizable outbreaks. . . . In Daegu, South Korea, just one woman, dubbed Patient 31, generated more than 5,000 known cases in a megachurch cluster.

Unsurprisingly, SARS-CoV, the previous incarnation of SARS-CoV-2 that caused the 2003 SARS outbreak, was also overdispersed in this way: The majority of infected people did not transmit it, but a few super-spreading events caused most of the outbreaks. MERS, another coronavirus cousin of SARS, also appears overdispersed, but luckily, it does not—yet—transmit well among humans.

This kind of behavior, alternating between being super infectious and fairly noninfectious, is exactly what k captures, and what focusing solely on R hides. . . .

Nature and society are replete with such imbalanced phenomena, some of which are said to work according to the Pareto principle, named after the sociologist Vilfredo Pareto. Pareto’s insight is sometimes called the 80/20 principle—80 percent of outcomes of interest are caused by 20 percent of inputs—though the numbers don’t have to be that strict. Rather, the Pareto principle means that a small number of events or people are responsible for the majority of consequences. This will come as no surprise to anyone who has worked in the service sector, for example, where a small group of problem customers can create almost all the extra work. . . .

To fight a super-spreading disease effectively, policy makers need to figure out why super-spreading happens, and they need to understand how it affects everything, including our contact-tracing methods and our testing regimes.

There may be many different reasons a pathogen super-spreads. Yellow fever spreads mainly via the mosquito Aedes aegypti, but until the insect’s role was discovered, its transmission pattern bedeviled many scientists. . . . Much is still unknown about the super-spreading of SARS-CoV-2. It might be that some people are super-emitters of the virus, in that they spread it a lot more than other people. . . .

In study after study, we see that super-spreading clusters of COVID-19 almost overwhelmingly occur in poorly ventilated, indoor environments where many people congregate over time—weddings, churches, choirs, gyms, funerals, restaurants, and such—especially when there is loud talking or singing without masks. For super-spreading events to occur, multiple things have to be happening at the same time, and the risk is not equal in every setting and activity. . . .

[Muge Cevik of the University of St. Andrews] identifies “prolonged contact, poor ventilation, [a] highly infectious person, [and] crowding” as the key elements for a super-spreader event. Super-spreading can also occur indoors beyond the six-feet guideline, because SARS-CoV-2, the pathogen causing COVID-19, can travel through the air and accumulate, especially if ventilation is poor. Given that some people infect others before they show symptoms, or when they have very mild or even no symptoms, it’s not always possible to know if we are highly infectious ourselves. We don’t even know if there are more factors yet to be discovered that influence super-spreading.

But we don’t need to know all the sufficient factors that go into a super-spreading event to avoid what seems to be a necessary condition most of the time: many people, especially in a poorly ventilated indoor setting, and especially not wearing masks. As Natalie Dean, a biostatistician at the University of Florida, told me, given the huge numbers associated with these clusters, targeting them would be very effective in getting our transmission numbers down.

Overdispersion should also inform our contact-tracing efforts. In fact, we may need to turn them upside down. Right now, many states and nations engage in what is called forward or prospective contact tracing. Once an infected person is identified, we try to find out with whom they interacted afterward so that we can warn, test, isolate, and quarantine these potential exposures. But that’s not the only way to trace contacts. And, because of overdispersion, it’s not necessarily where the most bang for the buck lies. Instead, in many cases, we should try to work backwards to see who first infected the subject.

Because of overdispersion, most people will have been infected by someone who also infected other people, because only a small percentage of people infect many at a time, whereas most infect zero or maybe one person. As Adam Kucharski, an epidemiologist, . . . explained to me, if we can use retrospective contact tracing to find the person who infected our patient, and then trace the forward contacts of the infecting person, we are generally going to find a lot more cases compared with forward-tracing contacts of the infected patient. [Those] will merely identify potential exposures, many of which will not happen anyway, because most transmission chains die out on their own. . . .

Even in an overdispersed pandemic, it’s not pointless to do forward tracing to be able to warn and test people, if there are extra resources and testing capacity. But it doesn’t make sense to do forward tracing while not devoting enough resources to backward tracing and finding clusters, which cause so much damage. . . .

Perhaps one of the most interesting cases has been Japan, a country with middling luck that got hit early on and followed what appeared to be an unconventional model, not deploying mass testing and never fully shutting down. By the end of March, influential economists were publishing reports with dire warnings, predicting overloads in the hospital system and huge spikes in deaths. The predicted catastrophe never came to be, however, and although the country faced some future waves, there was never a large spike in deaths despite its aging population, uninterrupted use of mass transportation, dense cities, and lack of a formal lockdown.

[Hitoshi Oshitani of Japan’s COVID-19 Cluster Taskforce] told me that in Japan, they had noticed the overdispersion characteristics of COVID-19 as early as February, and thus created a strategy focusing mostly on cluster-busting, which tries to prevent one cluster from igniting another. Oshitani said he believes that “the chain of transmission cannot be sustained without a chain of clusters or a megacluster.” Japan thus carried out a cluster-busting approach, including undertaking aggressive backward tracing to uncover clusters. Japan also focused on ventilation, counseling its population to avoid places where the three C’s come together—crowds in closed spaces in close contact, especially if there’s talking or singing . . .

Oshitani contrasts the Japanese strategy, nailing almost every important feature of the pandemic early on, with the Western response, trying to eliminate the disease “one by one” when that’s not necessarily the main way it spreads. Indeed, Japan got its cases down, but kept up its vigilance: When the government started noticing an uptick in community cases, it initiated a state of emergency in April and tried hard to incentivize the kinds of businesses that could lead to super-spreading events, such as theaters, music venues, and sports stadiums, to close down temporarily. Now schools are back in session in person, and even stadiums are open—but without chanting.

It’s not always the restrictiveness of the rules, but whether they target the right dangers. As [one scientist] put it, “Japan’s commitment to ‘cluster-busting’ allowed it to achieve impressive mitigation with judiciously chosen restrictions. Countries that have ignored super-spreading have risked getting the worst of both worlds: burdensome restrictions that fail to achieve substantial mitigation. The U.K.’s recent decision to limit outdoor gatherings to six people while allowing pubs and bars to remain open is just one of many such examples.”

Could we get back to a much more normal life by focusing on limiting the conditions for super-spreading events, aggressively engaging in cluster-busting, and deploying cheap, rapid mass tests—that is, once we get our case numbers down to low enough numbers to carry out such a strategy? Many places with low community transmission could start immediately. . . .

Science, Schmience

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From Paul Krugman’s newsletter:

Untitled

. . .  I’ve sometimes regretted having gone into economics, a field in which getting the story right all too often offends powerful players, who in turn intervene to prop up zombie ideas that should have died long ago.
 
But I also realized some time back that politics can and will intrude into any area of scholarly research where some people have strong motivations for getting the story wrong. This has obviously been the case for climate research, where an overwhelming scientific consensus has had to struggle against a whole industry of climate denial, which is almost entirely supported by fossil-fuel interests and has effectively taken over the Republican Party.
 
In fact, in some ways the climate scientists have had it worse than the economists; mainstream Keynesian economists (which is pretty much what I am) get a lot of abuse, but as far as I know none of us has had politicians trying to criminalize our work, the way Ken Cuccinelli, now a top official at the Department of Homeland Security, did to climatologist Michael E. Mann [ten years ago].
 
I used to think, however, that climate change was a subject uniquely vulnerable to anti-science propaganda and intimidation. After all, the effects of greenhouse gas emissions are invisible and gradual, taking decades to unfold; it’s always possible to mock the science because it happens to be snowing today, while accusing the scientists of taking jobs away from salt-of-the-earth coal miners.
 
Surely, I thought, it wouldn’t be that easy to politicize a science, to claim that all the experts were part of a vast conspiracy, in an area in which experts’ predictions could be validated and the conspiracy theorists revealed as phonies in a matter of weeks.
 
But I was wrong.
 
Epidemiology, like climatology — or for that matter economics — involves trying to model complex systems, so that no prediction ends up being exactly right. And the chains of cause and effect are long enough that the consequences of bad policy take some time to become completely apparent: Florida began reopening in early May, but Covid-19 deaths didn’t spike until July.
 
But we’re talking about weeks, not decades, and the story of the coronavirus is as clear as such things ever get.
 
Experts warned that a rush to resume business as usual, without social distancing and widespread use of face masks, would lead to a surge in new cases. The usual suspects on the right dismissed these concerns, insisting either that Covid-19 was a hoax or that its dangers were being greatly exaggerated by scientists who wanted to bring down Dxxxx Txxxx. Sunbelt states decided to believe the skeptics, not the scientists — and the result was a huge, deadly viral surge.
 
So that put an end to the politicization, right? Wrong. Not only are Txxxx officials still pressuring health experts to minimize the dangers, the top communications official at the Department of Health and Human Services accused his own agency’s scientists of “sedition.”
 
The moral here is that there’s no such thing as a safe subject when you’re dealing with people who have a totalitarian mind-set — and that is, in fact, what we’re dealing with. I suspect that in the early days of the Soviet Union plant geneticists imagined that they were working in a low-risk field; I mean, who would politicize that? In the end, however, thousands of them were sent to labor camps or executed for questioning the theories of Trofim Lysenko, a quack who somehow became one of Stalin’s favorites.
 
The fact of the matter is that we’re now struggling over where there’s even such a thing as objective truth. And staying out of politics is no longer an option for anyone.
 
Unquote.
 
I wouldn’t say we’re struggling over whether there’s objective truth. It’s real. What we’re struggling with are people who don’t respect it (like the chairman of the Republican Party who claimed that Biden’s record on the virus is worse than Txxxx’s). But Prof. Krugman is right about not staying out of politics. Voting — and supporting candidates in other ways — matters.
 
Which reminds me. Has anybody seen my Biden/Harris lawn sign? It still hasn’t been delivered. Maybe the crooked Postmaster General arranged for it to be confiscated?

Stunning Disclosures That Aren’t Breaking News

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Feeling the effects of a two weeks-long news vacation, I took a calculated risk and reviewed the subjects my two favorite columnists have written about. There wasn’t anything especially intriguing from Paul Krugman of The New York Times. But The Washington Post’s Paul Waldman got me reading with this headline: “Stunning new disclosures blow huge holes in Trump’s Potemkin facade”. 

From way back on September 9th, in other words, old news for you:

President Txxxx’s argument for reelection has two fundamental pillars. First, his handling of the pandemic was uniformly stupendous, vanquished the coronavirus to the point where it’s largely behind us and revealed that he has cared deeply about its victims all along.

Second, now that pandemic has been largely crushed, the most dire threat to ordinary Americans’ lives and communities is a terrifying specter of organized left-wing political violence that threatens civil collapse, and only Txxxx can stop it.

A host of new revelations — some involving Txxxx’s early understanding of the coronavirus threat, and others concerning his efforts to fabricate a leftist domestic terrorist menace — have blown up this multifaceted Potemkin facade.

The new revelations also illustrate in stark new detail just how corruptly Txxxx and his cronies have manipulated the levers of government to make all those illusions appear as truths.

[Note: The President Commits Murder in the First Degree]

We begin with the explosive disclosures in Post associate editor Bob Woodward’s new book about the Txxxx administration. With the U.S. death toll from covid-19 now approaching 200,000, Woodward’s conversations with Txxxx — all of which are on tape — reveal that the president knew all along how bad the pandemic would be.This is what Txxxx told Woodward on Feb. 7:

You just breathe the air and that’s how it’s passed. And so that’s a very tricky one. That’s a very delicate one. It’s also more deadly than even your strenuous flus. 
 This is more deadly. This is 5 percent vs. 1 percent and less than 1 percent. You know, so this is deadly stuff.

So to be clear, on Feb. 7 — when there were only 11 confirmed cases of covid-19 in the United States — Txxxx both emphasized the danger of the virus being passed through the air and explicitly declared he understood it was far more deadly than the flu.

Then on March 19, Txxxx told Woodward this:

Well I think Bob, really to be honest with you, I wanted to always play it down. I still like playing it down, because I don’t want to create a panic.

Txxxx admitted he knew the pandemic was worse than he said publicly, and that he deliberately minimized it to avoid panic. Remember, it has already been reported that the “panic” he feared was spooking the markets because that threatened his reelection.

Only one month later, on March 9, Txxxx dismissively compared the coronavirus with flu, claiming flu kills tens of thousands annually and that “life & the economy go on.” Txxxx downplayed the virus and said it was under control numerous times throughout February and March and beyond.

Even if a leader might in certain circumstances be justified in holding back information to avoid a panic, this was not one of those circumstances. In this case, there were very specific measures that we needed Americans to take to limit the virus’s spread, including social distancing, avoiding large indoor gatherings and wearing masks.

Every time the president said the pandemic wasn’t serious, he persuaded people not to take those measures. Not only that, he actively discouraged people from taking them, cheering on protests of lockdown orders in Democratic-controlled state capitals. He did this with the full knowledge that it would worsen the pandemic, leading to more deaths.

[The President Covers Up / Makes Up Threats to National Security]

Now on to the second revelation: House Democrats just released a complaint from a new whistleblower at the Department of Homeland Security. It makes a series of extraordinary charges about senior DHS officials seeking to manipulate intelligence to boost Txxxx politically.

The complaint from the whistleblower, Brian Murphy, a senior official at DHS’s Office of Intelligence and Analysis, claims intense pressure was brought to bear to hype the threat of leftist violence. The complaint says two top DHS officials — acting DHS secretary Chad Wolf and deputy Ken Cuccinelli — halted the distribution of a Homeland Threat Assessment because of how it “would reflect upon President Txxxx.”

“Two sections were specifically labeled as concerns: White Supremacy and Russian influence in the United States,” the complaint continues.

On Russian influence, the complaint says, Wolf ordered Murphy in May 2020 to report on efforts by China and Iran to interfere in our election, and to “cease providing intelligence assessments on the threat of Russian interference.”

Incredibly, the complaint says Murphy “would not comply with these instructions, as doing so would put the country in substantial and specific danger.”

The hyping of China and Iran seemed designed to false-equivalence away Russian interference, as Txxxx wants. Notably, we’ve already seen previous intelligence reports on foreign interference play this same deception game.

On white supremacy, the complaint describes discussions in May and June of 2020 with Cuccinelli about the DHS threat assessment at which this happened:

Mr. Cuccinelli stated that Mr. Murphy needed to specifically modify the section on White Supremacy in a manner that made the threat appear less severe, as well as include information on the prominence of violent “left-wing” groups. Mr. Murphy declined to make the requested modifications, and informed Mr. Cuccinelli that it would constitute censorship of analysis and the improper administration of an intelligence program.

That is simply remarkable — both in its downplaying of the right wing extremist threat and in its hyping of a leftist one.

Now remember that many of Txxxx’s top national security officials — including Attorney General William P. Barr, national security adviser Robert C. O’Brien and acting Customs and Border Protection chief Mark Morgan — have made public statements hyping the notion of a left-wing domestic terror threat. . . .

Much of [the Republican] convention was devoted to manufacturing illusions of an active, far-reaching and organized leftist domestic extremist threat.

Now we’re learning that intelligence may have been actively perverted to support this narrative in a manner that prompted a whistleblower to repeatedly object — and now to come forward.

All this comes as Barr is set to release a review of the origins of the Russia investigation that could discredit its findings, facilitating another round of Russian sabotage. Meanwhile, Txxxx’s intelligence officials are no longer providing in-person briefings on that sabotage, limiting members of Congress’s understanding about what Russia is doing and further easing their ability to meddle in our election. . . . 

Every time we think we’ve penetrated through to the very worst of this corruption, another layer gets peeled back, revealing still more. . . . 

We Are Probably Missing Something About the Virus (and Being Near Death)

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There is evidence that the virus is mostly spread through the air, but not the way we thought. “Droplets” might not be the main problem. They tend to fall from the air after traveling a few feet. It may be much smaller particles, what scientists call “aerosols”, that are the main culprit. It’s being suggested, therefore, that we treat the virus like cigarette smoke. Smoke isn’t much of a problem outdoors, but it is a big problem indoors, especially in poorly-ventilated spaces. 

Nature had an article about the issue last month. The following is from a Time magazine article written by a Colorado chemistry professor:

Inhaling a little whiff of “smoke” here and there is OK, but a lot of “smoke” for a sustained period of time and without a mask is risky. . . .  

We should continue doing what has already been recommended: wash hands, keep six feet apart, and so on. But that is not enough. A new, consistent and logical set of recommendations must emerge to reduce aerosol transmission. I propose the following: [avoid crowding, especially indoors; increase fresh air and ventilation; avoid close proximity for long durations, especially where there is talking, singing or yelling; and wear tight-fitting masks]. These are the important factors in mathematical models of aerosol transmission, and can also be simply understood as factors that impact how much “smoke” we would inhale.

[This] suggests that we should do as many activities as possible outdoors, as schools did to avoid the spread of tuberculosis a century ago, despite harsh winters. . . .

Second, masks are essential, even when we are able to maintain social distance. We should also pay attention to fitting masks snugly, as they are not just a parapet against ballistic droplets, but also a means to prevent “smoke” from leaking in through gaps. We should not [talk without masks], because we exhale aerosols 10 times as much when talking compared to breathing . . . 

In a fast-moving viral pandemic, scientific understanding will inevitably change as research catches up to the speed at which the virus spreads. However, it seems clear that aerosols are more important when it comes to transmitting COVID-19 than we thought six months ago—and certainly more important than public health officials are currently making them out to be. 

On a related topic, it’s true that a fewl countries have had more deaths per capita than the United States. Peru is the worst. Followed by Chile and Brazil. We’re fourth worst. With only 4% of the world’s population, the United States has had almost one-quarter of the world’s Covid-19 deaths. And we’re still losing close to 1,000 Americans every day.

The president wanted to build a wall to protect us from Mexico, a poor country. The US is a rich country, yet Mexico has fewer deaths per capita than we do. We should keep that in mind when we hear his supporters claim the president has done a good job keeping us safe and alive.

And given that we’re considering death, it’s interesting to note that what are called “near death experiences” (NDEs) aren’t always blissful. People sometimes report a feeling of peace, floating above their bodies, the disappearance of pain, etc. Scientific American published a good article on the subject in June. Aside from making the point that there are ways to induce similar experiences in a person who isn’t dying, the author suggests that all is not well in the world of NDEs:

They share broad commonalities . . . They might include meeting loved ones, living or dead, or spiritual beings such as angels; a Proustian recollection or even review of lifetime memories, both good and bad (“my life flashed in front of my eyes”); or a distorted sense of time and space. There are some underlying physiological explanations for these perceptions . . . 

NDEs can be either positive or negative experiences. The former receive all the press and relate to the feeling of an overwhelming presence, something numinous, divine. A jarring disconnect separates the massive trauma to the body and the peacefulness and feeling of oneness with the universe. Yet not all NDEs are blissful—some can be frightening, marked by intense terror, anguish, loneliness and despair.

It is likely that the publicity around NDEs has built up expectations about what people should feel after such episodes. It seems possible, in fact, that distressing NDEs are significantly underreported because of shame, social stigma and pressure to conform to the prototype of the “blissful” NDE.

Seven Months Later, What We Know About Covid-19 (and Don’t)

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Our president announced that New Zealand suffered a major surge of Covid-19 on Monday (“big surge in New Zealand, you know it’s terrible, we don’t want that”). They had nine new cases. The U.S. had 42,000. 

For somewhat more reliable information, see this informative summary from StatNews (the article has more about each item):

. . . In the time since Chinese scientists confirmed the rapidly spreading disease in Wuhan . . . an extraordinary amount has been learned about the virus, SARS-CoV-2, the disease it causes, Covid-19, and how they affect us.

Here are some of the things we have learned, and some of the pressing questions we still need answered.

What we know

Covid and kids: It’s complicated 

. . . Everything Covid is complex, and kids are no exception. While deaths among children and teens remain low, they are not invulnerable. And they probably contribute to transmission of SARS-CoV-2, though how much remains unclear. . . 

There are safer settings, and more dangerous settings

Research has coalesced on a few key points about what types of setting increase the risk that an infectious person will pass the virus to others. . . . 

People can test positive for a long time after they recover. It doesn’t matter 

There was a lot of angst a few months ago about some people who had seemingly recovered from Covid-19 infections continuing to test positive for the virus for weeks. Were they infectious? Should recommendations be changed for how long infected people should be isolated? It turns out it is an issue of testing. . . .

After the storm, there are often lingering effects 

Name a body part or system and Covid-19 has left its fingerprints there. . . . There are growing worries that these and other health effects will be long-lasting. . . .

‘Long-haulers’ don’t feel like they’ve recovered

We know they’re out there, but we don’t know how many, why their symptoms persist, and what happens next. . . . 

Vaccine development can be accelerated. A lot

An extraordinary amount of progress toward Covid-19 vaccines has been made, in record time. . . . 

People without symptoms can spread the virus

Whatever group you’re talking about, there are some key implications for the pandemic, and trying to rein it in. . . .

Mutations to the virus haven’t been consequential 

Coronaviruses in general do not mutate very quickly compared to other viral families. This is a good thing . . .  .

Viruses on surfaces probably aren’t the major transmission route

The general consensus now is that “fomites” — germs on surfaces — aren’t the major transmission route for Covid-19. . . .But it’s clear from lots of studies that surfaces around infected people can be contaminated with viruses and the viruses can linger. . . . 

What we don’t know

People seem to be protected from reinfection, but for how long? 

The thinking is that a case of Covid-19, like other infections, will confer some immunity against reinfection for some amount of time. But researchers won’t know exactly how long that protection lasts until people start getting Covid-19 again. So far, despite some anecdotal reports, scientists have not confirmed any repeat Covid-19 cases. . . .

What happens if or when people start having subsequent infections? 

Given that most respiratory viruses are not “one-and-done” infections — they don’t induce life-long immunity in the way a virus like measles does — there is a reasonable chance that people could have more than one infection with Covid-19. . . .

How much virus does it take to get infected? 

Whether you become infected or not when you encounter a pathogen isn’t just a question of whether you’re susceptible or immune. It depends on how much of the virus (or bacterium) you encounter. . . .

How many people have been infected?

There have been 21 million confirmed cases of Covid-19 around the world, and 5.3 million in the United States. Far more people than that have actually had the virus. . . .

It’s not clear why some people get really sick, and some don’t 

The sheer range of outcomes for people who get Covid-19 — from a truly asymptomatic case, to mild symptoms, to moderate disease leading to months-long complications, to death — has befuddled infectious disease researchers. . . .